1. Field of the Invention
The present invention relates to a sheet stacking apparatus that is connected to a sheet discharge section of an image forming apparatus or the like.
2. Discussion of the Background
There is used a sheet stacking apparatus that includes a sheet discharge tray capable of raising and lowering in a vertical direction a stacking paper sheet (hereinafter referred to as a sheet) discharged from an image forming apparatus or the like and a sheet discharge roller, which is located at an upper part of the sheet discharge tray, for discharging the sheet to the sheet discharge tray. The sheet discharge tray of the sheet stacking apparatus receives the sheet from the sheet discharge roller at a standard sheet receiving position that is located at a predetermined downward distance away from the sheet discharge roller.
The aforementioned standard sheet receiving position is set in consideration of a normal sheet that is not stapled (hereinafter referred to as a non-stapled sheet) and is not curled, and even a stapled set of sheets having a mounded portion due to a thickness of staples is not apparently accounted for.
For example, as shown in FIG. 10, when a sheet discharge tray 12 stays at a standard sheet receiving position, the position of the sheet discharge tray 12 is at a predetermined distance L lower than a standard position relative to a sheet discharge roller 3, such as measured at a nip portion of the sheet discharge roller 3, for example. More particularly, the distance L is defined as a distance between the nip portion of the sheet discharge roller 3 and an upper surface of the sheet discharge tray 12 when the sheet discharge tray 12 is not loaded with the sheets, or between the nip portion of the sheet discharge roller 3 and an uppermost surface of the sheets on the sheet discharge tray 12 when the sheet discharge tray 12 is loaded with the sheets. Such a distance L is controlled to be constant by moving the sheet discharge tray 12 by a controller.
Actually, a height of the sheets stacked on the sheet discharge tray 12 is read by a sensor, and the distance L is controlled in accordance with the reading result by the sensor. The distance L accordingly may be within a certain range depending on an accuracy of the sensor.
When the sheet discharging tray 12 is not loaded with the sheets, the standard position for receiving the sheet is at the distance L beneath the nip portion of the sheet discharge roller 3. The sheet discharge tray 12 receives a first sheet discharged from the sheet discharge roller 3 at the standard sheet receiving position. The sheet discharge tray 12 has an open end side (downstream relative to the sheet after being discharged), which is positioned higher than a base end side (upstream relative to the sheet after being discharged) in a vertical direction. The sheet discharge roller 3 conveys and discharges the sheet toward the sheet discharging tray 12 with a certain degree of momentum.
After being discharged out of the sheet discharge roller 3 onto the sheet discharge tray 12, the sheet slides down along a slope of the sheet discharge tray 12 (to be more clear, the sheet slides down along the slope of the sheet stacked on the sheet discharge tray 12 in a switchback manner) by its own weight. The sheet is then stopped by a rear fence which is mounted on the base end side of the sheet discharge tray 12.
As the sheets are successively stacked on the sheet discharge tray 12, a height of a top surface of the sheets on the sheet discharge tray 12 increases. During this stacking operation, if the position of the sheet discharge tray 12 is not changed, a distance between the top surface of the stacked sheets and the sheet discharge roller 3 becomes smaller than the distance L which is made when the sheet discharge tray 12 is not loaded with the sheets. If such a distance becomes too small, the sheets discharged on the sheet discharge tray 12 can no longer be aligned properly. To avoid this problem, the sheet discharge tray 12 is controlled to lower each time when a sheet is stacked thereon so as to maintain the distance L within an appropriate range, and the sheets can be aligned.
Although an accurate and continuous control of the distance L is needed to be considered, the controller performs an intermittent control in which the sheet discharge tray 12 is lowered each time as part of the sheet stacking operation by an amount of distance so that the height of the top surface of the stacked sheets does not affect the alignment of the stacked sheets. The distance L is thus controlled to be constant within a predetermined allowance. The sheet discharge tray 12 repeats the above-described intermittent-lowering operation in accordance with a sheet stacking amount to receive the sheets. In such a case, the standard sheet receiving position is the top surface of the sheet discharge tray 12 (if the sheet discharge tray 12 is loaded with the sheets, the top surface of the sheets) which is positioned at the distance L, including the above-mentioned predetermined allowance, downward from the nip portion of the sheet discharge roller 3.
When the sheet stacking apparatus is combined with a staple device, the sheet discharge tray 12 may receive various differently-formed sheets from the sheet discharge roller 3. In some cases, the sheet discharge tray 12 may receive only non-stapled sheets or only a set of stapled sheets, or both non-stapled sheets and a set of the stapled sheets in a mixed fashion, depending upon the selected mode related to the sheets. In addition, such a sheet stacking status will be maintained until the sheets are removed from the sheet discharge tray 12.
The sheet stacked on the sheet discharge tray 12 generally has a curl like a convex or concave shape, regardless of being stapled or not and of an amount of the sheets. For example, when the non-stapled sheet having a convex-shaped curl is stacked one after another on the sheet discharge tray 12, the curls of the sheets are accumulated. This accumulation of curls causes the sheets to partly form a big mounded portion therein.
If such a situation arises, even though the height of the sheet discharge tray 12 is controlled around the standard sheet receiving position, the newly discharged sheet is caught at the aforementioned mounded portion of the previously stacked sheets on the sheet discharge tray 12, and the trailing edge of the sheet cannot slide down toward the rear fence. Hence, the sheets will be improperly aligned on the sheet discharge tray 12. Accordingly, the sheet that is caught on the mounded portion of the stacked sheets may be displaced at the leading edge side thereof toward the open end side of the sheet discharge tray 12.
When the sheet discharge tray 12 is loaded with the thus-displaced sheets, assuming that the next sheet is further discharged from the sheet discharge roller 3, the next sheet wraps over the aforementioned displaced sheets with the leading edge side thereof. Furthermore, the above-mentioned next sheet moves together with the displaced sheet by friction force. As a result, the misalignment of the stacked sheets occurs and, in an extreme case, the sheet may fall down from the sheet discharge tray 12.
Such a misalignment of the sheets caused by the curl also occurs in a case of the stapled sheets. For example, when the stapled sheets are discharged, since the stapled sheets are formed in a set and have a larger rigidity than the non-stapled sheet, the stapled sheets set is stopped by another set of stapled sheets that are stuck over the mounded portion due to the curl and improperly aligned. The stapled sheets set then pushes other sets of stapled sheets to fall down from the sheet discharging tray 12. In some cases, the leading edge of the next set of stapled sheets is stopped by the mounded portion of the stacked sets of the stapled sheets and convolve with the sheet discharge roller 3 nip or become misaligned.
The standard sheet receiving position of the sheet discharge tray 12 of the typical sheet stacking apparatus is determined in consideration of using ordinary sheets without curl or sheets which are stapled at a position which does not face the sheet discharge roller 3. Accordingly, a detecting position on the top surface of the stacked sheets for a sensor for setting and controlling the standard sheet receiving position of the sheet discharge tray 12 is deviated from the position that faces the sheet discharge roller 3.
This deviation of the detecting position may cause a problem of sheet stacking in some cases. That is, when a set of the sheets which is stapled at a rear end side thereof, or at a sheet discharge roller 3 side, is successively stacked, or when a sheet having a curl at the sheet discharge roller 3 side is successively stacked, the distance L may be maintained between the top surface of the stacked sheets and the sheet discharge roller 3 only at the beginning of a stacking operation of the sheets. Then, the stacked sheets may start to form a large mound with accumulated curls, in a convex direction as mentioned earlier, with increasing number of stacked sheets. In a case of the concave-shaped sheets, as also mentioned earlier, the curled portion thereof may largely be lifted up. Thereby, in an area that includes the top surface of the lifted-up portion of the stacked sheets and its vicinity, the distance between such an area and the sheet discharge roller 3 becomes smaller than the distance L.
In a case of a set of stapled sheets, since the thickness of the staples is accumulated in the number of a set of the stapled sheets, the sheets that are stuck by a set of stapled sheets are largely lifted up or mounded in a convex shape. Also, the distance between the top surface of such a displaced portion of the stacked sheets and the sheet discharge roller 3 becomes smaller than the distance L.
Thus, the distance between the top surface of the stacked sheets and the sheet discharge roller 3 becomes small at both cases of the curled sheet and a set of the stapled sheets. Then, the top surface of the curled sheet and the rear end portion of the vicinity of the top surface of the curled sheet or the staple portion of the stapled sheets eventually starts to rub with the sheet discharge roller 3. If such a rubbing occurs, stacking performance deteriorates due to convolvement of the sheets by the sheet discharge roller 3, or the load added to the sheet discharge roller 3 increases. In some cases, the sheet itself is damaged. In an excessive case, the sheet discharge roller 3 is scraped by the convolved sheets, or rotation of the sheet discharge roller 3 is stopped due to the excessive load caused by the convolved sheets. Furthermore, when a set of stapled sheets is discharged into the sheet discharge tray 12, the tip portion thereof hits and pushes the sheets already stacked at a portion mounded due to the staples, eventually pushing the sheets off of the sheet discharge tray 12 such that the sheets fall on the floor.